This invention relates to the spinning of synthetic filaments. More particularly, this invention relates to a novel apparatus for the spinning of synthetic filaments utilizing a spinneret nozzel designed to establish an essentially constant extensional strain rate.
In connection with the production of filaments from man-made fiber forming materials, it has been common to utilize spinnerets including a plurality of exit passageways in the form of nozzles or orifices machined in a spinneret plate. A liquid comprising a polymer melt or a solution of a polymer in an appropriate solvent is extruded through these nozzles in the filament forming process.
As will be appreciated, the profile of such spinneret nozzles has been the subject of considerable efforts. In this connection, reference may be had to U.S. Pat. No. 3,537,135 (issued Nov. 3, 1970), U.S. Pat. No. 3,303,530 (issued Feb. 14, 1967), U.S. Pat. No. 3,210,451 (issued Oct. 5, 1965), U.S. Pat. No. 3,227,009 (issued Jan. 4, 1966), and U.S. Pat. No. 3,174,183 (issued Mar. 3, 1965) for disclosures of spinnerets with exit passageways having a variety of profiles.
An important consideration in fluid flow through a spinneret passageway of any given profile is that of fluid pressure drop. The pressure drop across a spinneret can be expressed as the sum of the viscous dissipation in the passageway and the entrance and exit flows, the pressure drop due to kinetic energy effects, and that due to storage of elastic energy.
Another important consideration in spinneret fluid flow is that of extrudate swell or die swell, i.e. the ratio of the fiber cross-sectional area after it passes the exit orifice of the spinneret exit passageway to the cross-sectional area of that exit orifice. Die swell has a direct effect on draw ratios and fiber elongation.
For example, in the case of spinning polyethylene terephthalate (PET), increased extrudate swell resulting from increased spinning productivity would produce decreases in after-draw ratio. Similarly, in connection with production of dry spun fibers from cellulose triacetate (CTA), increased extradate swell would reduce fiber elongation.
It has been found that extrudate swell in connection with flow through a capillary is inversely related to the ratio of the capillary length to its diameter. However, under high spinning productivities, attempts to reduce extrudate swell by increasing that ratio may produce an excessive pressure drop due to viscous dissipation.
Since costs associated with the operation of spinning equipment are considerable, adequately high spinning productivity must be maintained.
It would, therefore, be highly desirable to provide a novel spinning apparatus based upon a passageway profile designed to advantageously accomodate desirable spinning productivity consistent with acceptable pressure drop and extrudate swell. Accordingly, it is a general object of the present invention to provide such a novel spinning apparatus.
In spinning operations, one important viscosity consideration is that of fluid pressure drop in spinneret flow due to viscous dissipation. Under high productivity spinning conditions, viscous pressure drop often becomes excessive. As such, viscous pressure drop to some extent constitutes a limiting factor on spinning productivity. Since the costs associated with the operation of spinning equipment are considerable, it would be highly desirable to provide for increased spinning productivity by minimizing viscous pressure drop or to provide for a viscous pressure drop consistent with a given spinning productivity.
In realizing this object, the present invention departs from those prior art approaches to design of spinneret nozzle profiles which have been governed primarily by direct viscosity considerations, and perhaps, to some extent, inertial and turbulence considerations.
In this connection, the present invention embodies a recognition that spinneret flow for fluids generally classified as "viscous", such as PET melts or CTA solutions, is dominated by "elastic" forces. More particularly, according to the present invention a nozzle profile is provided so as to establish an essentially constant extensional strain rate, viz. a constant change in velocity with respect to the incremental distance of fluid travel (an elastic force consideration). In this fashion the flow condition for the liquid is such as to provide for a low total pressure drop. At the same time adequate spinning speeds may be employed with acceptable die swell.
The essentially constant extensional strain rate leads to a low total pressure drop through several mechanisms. First, since the elastic pressure drop contribution to the total pressure drop is governed by the maximum extensional strain rate, a low constant extensional strain rate may be chosen to reduce that contribution as compared with prior art nozzle profiles. Secondly, as hereinafter more fully described, at a chosen constant extensional strain rate, the viscous pressure drop is minimized. In addition, the gentle streamline entry flow provided by the nozzle profile of the present invention is believed to reduce dissipation due to vena contracta type eddies.
It may be here noted that an apparently similar profile has been proposed for an entirely different purpose, namely, the design of a minimum length die to be used for extruding on a wire, plastics which are subject to melt fracture. (See Ferrari U.S. Pat. No. 3,382,535, issued May 14, 1968.) Considerations directed toward elimination of melt fracture and minimizing die length in the environment of extruding plastics on a wire are unrelated to the present invention, in that polymers involved in the contemplated spinning operations do not exhibit any significant melt fracture in the range of operating conditions selected in contemplation for practice of the present invention.
Consistent with the foregoing, it is, therefore, a further object of the present invention to provide a novel spinning apparatus wherein an essentially constant extensional strain rate is established for flow through a spinneret nozzle.
It is a further object of the present invention to provide such a novel spinning apparatus wherein adequate spinning speeds may be employed with acceptable die swell.
It is yet another object of the present invention to provide such a novel spinning apparatus wherein a low total pressure drop is produced.
It is still another object of the present invention to provide such a novel spinning apparatus which minimizes breaking of filaments in a spin line, and thus minimizes the occurence of incomplete packages. In this connection, it is believed that breaking of filaments in a spin line might, in some respects, be attributable to faults or impurities, in the spinning material. The particular improved spinneret flow produced according to the present invention is thought to minimize the adverse consequences attributable to such faults or impurities.